As the light-emitting efficiency is increased and the cost of manufacturing is decreased, the dream for solid lighting device to replace the traditional lighting device may come true in recent years. Generally speaking, white light is provided by using blue light emitted from a light-emitting diode (LED) formed by a nitride semiconductor to excite the yellow phosphor. A nitride semiconductor including nitrogen (N) is a prime candidate to make a short-wave light-emitting device because its band-gap is sufficiently wide. Among other things, GaN-based compound semiconductors have been researched and developed particularly extensively. As a result, blue LEDs, green LEDs, and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products.
FIG. 1 shows the structure of the conventional nitride semiconductor LED 1. The conventional nitride semiconductor LED 1 comprises an n-type semiconductor 2, a multiple quantum well structure 4 and a p-type semiconductor layer 6. Usually, the n-type semiconductor 2 is an n-type GaN, and the p-type semiconductor layer 6 is a p-type GaN. In order to raise the light-emitting efficiency of the nitride semiconductor LED 1, the multiple quantum well structure 4 of the nitride semiconductor LED comprises a plurality of barrier layers 41 and well layers 42 different from the barrier layers 41. The plurality of the barrier layers 41 and the well layers 42 are alternately stacked with each other to form the traps to capture the electrons and holes to increase the combination efficiency thereof. Generally, the plurality of the barrier layers 41 comprises Ga and N, and preferably is GaN. The plurality of the well layers 42 comprises In, Ga, and N, and preferably is InGaN. Nevertheless, the lattice constant of the GaN does not match the lattice constant of the InGaN so forms the strain in the multiple quantum well structure 4. The strain causes the defects in the multiple quantum well structure 4 and decreases the light-emitting efficiency.